Anodizing is a process which has long been used to protect the surface of aluminum components from corrosion. The process consists of making a component anodic in an acidic solution. A typical anodizing process consists of degreasing, pickling/etching (or brightening), desmutting, anodizing, sealing and aging steps.
The process of anodization leads to the formation of a porous oxide layer on the aluminum surface which may have a thickness in the range of 3 to 25 microns depending on the field of application. Because the oxide layer is porous, it is necessary to seal the pores to prevent corrosion. One method uses hot water (typically used at boiling point) for sealing porous oxide layers. However, the required immersion time to achieve complete sealing of the surface is between 2 and 3 minutes per micron of oxide coating, which can lead to overall lengthy immersion times. Additionally, using hot water for sealing is not energy efficient and there are obvious safety hazards involved with the use of boiling water. The oxide layer is often not homogeneous on aluminum alloys with high amounts of silicon. Due to the non-uniformity of the oxide layer, such alloys cannot be successfully treated using hot water because the resulting corrosion performance will not be adequate.
In order to address the problems associated with hot water sealing processes, low temperature sealing processes have been developed using nickel salts, typically using nickel fluoride. These processes operate at low temperatures, typically less than 30° C., and involve a contact time of about 1 minute per micron of oxide on the aluminum surface. The sealing process is thought to be accomplished via the formation of a complex of nickel aluminum-fluoride salt in the pores of the anodized coating.
Nickel based sealing processes have obvious advantages in terms of production throughput and energy efficiency. Furthermore, using nickel based sealing processes provides good corrosion resistance, especially for those aluminum alloys higher in silicon. However, the use of nickel is becoming increasingly restricted due to its carcinogenic properties; therefore a low temperature sealing process that does not contain nickel is desirable for providing corrosion resistance on anodized aluminum surfaces. Additionally, because of the toxicity of nickel, measures must be taken to carefully treat the wastewater from nickel based sealing processes, which can be very expensive.
There have already been attempts to produce nickel-free, low temperature sealing systems, but none of these at present effectively addresses the problems associated with treatment of high silicon alloys. For example, Canadian Patent 2,226,418 to Koerner et al. proposes the use of a lithium fluoride based immersion process (optionally containing molybdate, vanadate or tungstate ions) prior to a conventional hot sealing process (80-100° C.). The process is claimed to reduce the immersion time required in the hot process and provide effective sealing of anodized metals. However, temperatures in excess of 80° C. are still required. U.S. Pat. No. 4,786,336 to Schoener et al. describes a low temperature (40° C.) process using a composition based on fluoro-zirconates or fluoro-tungstates in combination with silicate. However, this process does not produce satisfactory results on anodized aluminum alloys with high silicon content.
There are many industrial applications in which aluminum alloys have higher than 1% silicon where corrosion resistance is critical. Brake calipers are an excellent example of an aluminum alloy component that may comprise a high percentage of silicon, where a sufficiently sealed surface will be paramount to the corrosion resistance of the final product. Accordingly, there is a need for a nickel-free, low temperature sealing process suitable for all anodized aluminum alloys including high silicon alloys.